• analytical chemistry
• azurite-based paint
• copper
• mixed media paint
• oil curing
• oil paint
• pre-polymerized siccative oil
• protein coated pigment paint
• siccative oil
• tempera grassa

This study investigates the polymerization mechanisms of azurite, a basic copper carbonate known to be the most widely used blue pigment in European art throughout the Middle Ages and the Renaissance, in oil-based systems using a multi-analytical approach. Azurite interacts with drying oils, leading to instability and inhomogeneous cross-linking. The role of Cu(II) in oxidative polymerization was examined by comparing raw and pre-polymerized linseed oils across three painting techniques: pure oil, tempera grassa, and protein-coated pigment (pcp) paints, two mixed media paint techniques. NMR relaxometry, GC-MS, DSC, FTIR, TGA, and colorimetry were used to assess drying behavior and stability.
Results show that in oil paint, raw oil promotes localized Cu(II)-induced peroxidation, causing exudation and phase separation, while pre-polymerized oil enables more controlled curing. In pcp paints, enhanced Cu(II) extraction by the proteinaceous layer accelerates drying and homogenizes the film. In tempera grassa, Cu(II) dispersion in the aqueous-protein phase dictates curing, making oil type irrelevant. These findings highlight how oil properties and additional binders influence azurite stability, confirming that proteinaceous components are essential with raw oils but unnecessary with pre-polymerized formulations.